The invention relates to a method and a system for controlling vehicle speed in a vehicle equipped with brake cruise control when the vehicle is travelling downhill. The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to a heavy-duty vehicle, the invention is not restricted to this particular vehicle, but may also be used in other vehicles such as articulated haulers, wheel loaders, and other working machines or automobiles.
Driver assistance systems for automatically controlling the speed of a vehicle, often termed cruise control systems, allow a driver to specify a selectable driving speed via actuating means. The set speed is then maintained independently of the operation of an accelerator pedal of the vehicle. The speed can be set by the driver and can also be retained independently of the driving situation, such as uphill or downhill driving. The vehicle will then use suitable means such as auxiliary brakes or service brakes to maintain the set speed.
DE 10 2006 051 179 A1 (corresponding to U.S. Patent App. Pub. US2007/0099579) relates to a method for automatic downward velocity control, commonly known as brake cruise control, which is capable of absorbing energy induced by the inclination automatically. In this way the driver must no longer be active to maintain a substantially constant traveling speed. The downward velocity control is triggered when the accelerator pedal is released, and the vehicle begins to accelerate downhill. The vehicle speed is controlled towards target vehicle speed, which is a desired predetermined constant vehicle speed set by the driver.
When travelling downhill in a vehicle with brake cruise control, it is not always possible to maintain the brake set speed by using auxiliary brakes only. Reducing the vehicle speed and/or manually resetting the brake set speed is often necessary to avoid over-using the service brakes in long or steep downhill conditions. Extended periods of service brake actuation can cause the brakes to overheat, which can cause the brakes to wear at an increased rate and may even damage the brakes.
In a conventional vehicle this has to be done by the driver by manually adjusting the brake set speed and pressing the brake pedal to actuate the service brakes to reduce the vehicle speed down to the desired speed. At the end of the downhill road section, the brake set speed has to be manually adjusted by the driver back to the originally set value.
It is desirable to provide a method and a system for controlling vehicle speed in a vehicle equipped with brake cruise control which eliminates the above problems. The invention allows, according to an aspect thereof, automatic control of the speed of a vehicle requiring a minimum of intervention and providing a high level of comfort for the driver.
It is desirable to provide a method and a system for controlling vehicle speed in a vehicle equipped with brake cruise control.
In this context, the term “service brake” is used to denote the normal, or main brakes used in heavy-duty vehicles, usually operated by the driver depressing a service brake pedal. The service brakes are often, but not necessarily, air brakes operated by compressed air controlling friction brakes provided on each wheel to be braked on the vehicle. The term “auxiliary brakes” is used to denote additional means for assisting braking of the vehicle. Auxiliary brakes are used in addition to the service brakes and allow for prolonged brake application without requiring assistance from the service brakes. In this way, wear and unnecessary heating of the service brakes can be avoided. Such auxiliary brakes can comprise an exhaust valve, for activating an exhaust brake, engine brake valves, for activating a compression release engine brake, a fluid retarder, for generating a braking force on a propeller shaft, or means for driving a compressor or generator to generate a braking force. For electric and hybrid electric vehicles the auxiliary brake can comprise a generator, a motor-generator or other suitable means.
The invention, according to an aspect thereof is applicable to vehicles provided with a brake cruise control system. According to an aspect of the invention the system can be used for vehicles provided with internal combustion engines, as well as for electric and hybrid electric vehicles. This cruise control system can be enabled by the driver prior to or during downhill travel. When the actual, or current vehicle speed exceeds a brake set speed set by the driver-, an auxiliary brake, such as an engine brake, a motor-generator, or similar, will engage to maintain that speed within the limits of the auxiliary brake.
According to a first aspect of the invention, a method for controlling vehicle speed in a vehicle equipped with brake cruise control when the vehicle is travelling downhill is provided. The method comprises the steps of;                driving a vehicle downhill with the brake set speed set to a first brake set speed;        detecting a current vehicle speed;        automatically applying a brake torque using at least an auxiliary brake to maintain the first brake set speed;        detecting a manual application of a vehicle service brake, in order to decrease the vehicle speed;        
In this context, the steps of detecting a current vehicle speed and automatically applying a brake torque using at least an auxiliary brake to maintain the first brake set speed are actions performed by the brake cruise control without input from the driver.
If a control unit provided in the vehicle detects manual application of the service brake, i.e the driver is applying the service brake, then the control unit is automatically arranged to perform the steps of:                setting the brake set speed to a second brake set speed that is lower than the first brake set speed; and        applying a brake torque using at least the auxiliary brake if a detected current vehicle speed exceeds the second brake set speed.        
An advantage of this method is that the driver is not required to perform any action, apart from briefly applying the brake pedal, to reduce the speed of the vehicle to a lower brake set speed. No additional switches or controls need to be actuated and no new set speed has to be selected by the driver. A reduction of the manual actions required for this purpose allows the driver to concentrate on the road and improves the driving comfort.
According to a first alternative example, the control unit is automatically arranged to set the second brake set speed at a level where the second brake set speed can be maintained by the auxiliary brake.
According to a second alternative example, the control unit is automatically arranged to set the second brake set speed at the vehicle speed detected when the driver released the service brakes.
The method is arranged to repeat the above steps until it is detected that the downhill condition has ended, i.e. the slope is levelling out and normal cruise control can be resumed. Hence, when travelling downhill with the brake set speed to the second brake set speed it is possible to perform further speed reductions if required. Should the control unit detect a further manual application the service brake, for instance caused by a steeper than expected slope, then the brake set speed is set to a third brake set speed that is lower than the second brake set speed.
When the control unit detects that the downhill condition has ended then the brake cruise control brake set speed is restored to the first brake set speed and one or more upshifts are performed. According to a first alternative example this step can be performed automatically in response to one or more signals transmitted to the control unit by an inclination sensor, an accelerometer, an on-board topographic database, a GPS-receiver, or a similar suitable device.
According to a second alternative example the brake set speed is restored to the first brake set speed when the driver issues a resume command. Such a resume command can involve the driver actuating an accelerator means or a resume switch on the dashboard, or on or adjacent the steering wheel. The resume command is detected by the control unit, which restores the first brake set speed
An advantage of the latter method steps is that the driver is not required to perform any action, or alternatively a single action involving a brief application of the accelerator pedal or actuating one switch, to resume the first brake set speed of the vehicle. No additional switches or controls need to be actuated and no new/earlier set speed has to be selected by the driver. This further reduces the manual actions required, which allows the driver to concentrate on the road and improves the driving comfort.
According to a second aspect of the invention, a vehicle speed control system in a vehicle equipped with brake cruise control when the vehicle is travelling downhill is provided. The system comprises a sensor arranged to detect a current vehicle speed; a service brake;
at least one auxiliary brake; and a control unit having a user selectable input for receiving a brake set speed, said control unit being arranged to compare the current vehicle speed with a first brake set speed and control the auxiliary brakes to maintain the first brake set speed. The first brake set speed can be set once by the driver and is then stored in the control unit for future use. Alternatively, the system can prompt the driver for an input, e.g. by displaying the currently stored set speed value, and if no input is provided by the driver then the stored first brake set speed is used. The speed sensor can be a separate sensor provided for this purpose, a sensor forming part of an ABS arrangement in the service brake, or a virtual sensor where the vehicle speed is calculated from suitable engine and transmission parameters. The control unit is arranged to detect if the service brake is applied to retard the vehicle speed. When a manual application of the service brakes is detected, the control unit is automatically arranged to set the brake set speed to a second brake set speed that is lower than the first brake set speed. The control unit is subsequently arranged to apply a brake torque using at least the auxiliary brake if the current vehicle speed exceeds the second brake set speed. In this way, the auxiliary brake is used as the primary source of brake torque. Only when the auxiliary brake or brakes cannot maintain the second brake set speed will the service brake be actuated by the control unit. Automatic actuation of the service brakes should only be performed during short periods of time of intermittently the driver may be notified if prolonged automatic application of the service brakes is carried out.
According to a first alternative example the control unit is arranged to set the second brake set speed at a level where the brake set speed can be maintained by the auxiliary brake or brakes.
According to a second alternative example the control unit is arranged to set the second brake set speed at the vehicle speed detected when the driver released the service brakes.
When the control unit detects that the downhill condition has ended then the brake cruise control brake set speed is restored to the first brake set speed. According to a first alternative example the control unit is arranged to restore the brake set speed to the first brake set speed when the control unit has detected that the downhill condition has ended. The control unit is arranged to perform this action in response to one or more signals transmitted to the control unit by an inclination sensor, an accelerometer, an on-board topographic database, a GPS-receiver, or a similar suitable device.
According to a second alternative example the control unit is arranged to restore the brake set speed to the first brake set speed when the driver issues a resume command. Such a resume command can involve the driver actuating an accelerator means or a resume switch on the dashboard, or on or adjacent the steering wheel.
According to a third aspect of the invention, a computer program is provided comprising program code means for performing the above-mentioned method steps for controlling vehicle speed in a vehicle equipped with brake cruise control when said program is run on a computer. The invention further relates, according to an aspect thereof, to a computer readable medium carrying a computer program comprising program code means for performing the above-mentioned method steps for controlling vehicle speed in a vehicle equipped with brake cruise control when said program product is run on a computer. Finally, the invention further relates, according to an aspect thereof, to a control unit for controlling vehicle speed in a vehicle equipped with brake cruise control, the control unit being configured to perform the steps of the above-mentioned method.
By the provision of a method and a system which involve automatic control of a brake cruise control system, the advantage is that the driver can be relieved of a number of the manual actions by transferring control of the brake cruise control system to a control unit. With the brake cruise control system active, the driver merely needs to apply the service brakes to indicate that a speed reduction is desired. When the service brakes are released, the control unit will automatically set the brake set speed to a lower value. This lower value is retained to the end of the downhill road section, or until the driver indicates a further speed reduction. At the end of a downhill section the control unit can be arranged to automatically reset the brake set speed to the initial brake set speed. Alternatively, the driver can indicate that the initial brake set speed is to be reset by issuing a resume command, for instance by actuating a resume switch or by depressing an accelerator pedal. In this way the number of manual actions required by the driver can be reduced to an absolute minimum, allowing the driver to concentrate on the road and improving the comfort for the driver.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.